1. Field of the Invention
The present invention relates to an optical pickup apparatus for reproducing signals recorded in optical disks and, in particular, to an optical pickup apparatus which can reproduce signals from a plurality of kinds of optical disks having thickness values different from each other.
2. Description of the Prior Art
Optical disks have been used as various recording media since they have a high recording density and a large capacity while being excellent in random access characteristics.
Accordingly, as the kinds of the optical disks, there have been known various types such as CD (compact disk), DVD (digital video disk), and LD (laser disk) according to their uses and fields of utilization.
Regardless of the kinds of the optical disks, optical pickup apparatuses for reproducing signals from these various types of the optical disks have substantially the same configuration such as that shown in FIG. 11.
Namely, in general, in the optical pickup apparatus, a laser beam 102 output from a semiconductor laser 101 is focused on a recording surface 106A of an optical disk 106 by an objective lens 105 by way of a half mirror 103 and a collimator lens 104. Light returning from the recording surface 106A enters a four-divided photodiode 107 by way of the objective lens 105, the collimator lens 104, and the half mirror 103 so that a data signal and an error signal (e.g., focus error signal and tracking error signal) are obtained. Also, in response to this error signal, the above-mentioned objective lens 105 is moved up and down as well as left and right to effect focus control and tracking control.
As mentioned above, since the optical pickup apparatus has substantially the same configuration with respect to various kinds of optical disks, there can be constructed a so-called compatible player in which a single optical pickup apparatus is used for reproducing signals from various kinds of optical disks. Such a compatible player is convenient in that various kinds of player apparatuses do not have to be prepared so as to correspond to the kinds of the optical disks.
Meanwhile, thickness or NA of these optical disks often varies according to their kinds. In particular, their thickness may vary greatly. For example, though CD and DVD have the same diameter of 120 mm, the thickness of the CD is standardized at 1.2 mm, whereas the DVD has a standard thickness of 0.6 mm.
Also, the LD has a configuration in which two sheets of single-plate disks each having a thickness of 1.2 mm are bonded together with their recording surfaces facing inside. The thickness of its half side is 1.2 mm which is the same as the thickness of the CD. However, while the CD and DVD have a protective plate made of PC (polycarbonate) resin in general, the LD has a protective plate made of acrylic resin in general. As a result, the refractive indexes of these resins differ from each other, thereby making the optical thickness of the LD different from that of the CD and DVD.
Therefore, when a single optical pickup is commonly used for a plurality of kinds of optical disks having geometric or optical thickness values different from each other, a remarkable aberration may occur due to their difference from the thickness of a basically designed optical disk, thereby making it difficult to effect focusing.
Accordingly, there have been proposed techniques in which, when the same optical pickup apparatus is commonly used for a plurality of optical disks whose geometric or optical thickness values differ from each other, an optical transparent plate for compensating for the this thickness difference is inserted into and retracted from the space between the optical disk and the objective lens (see Japanese Unexamined Patent Publication No. 3-54740, U.S. Pat. No. 5,097,464, and, if necessary, U.S. Pat. No. 5,235,581).
Nevertheless, aberration, such as spherical aberration in particular, may not always be improved when the refractive index or thickness of the optical transparent plate is determined simply on the basis of the difference in geometric or optical thickness.
Namely, since light beams are incident on the optical transparent plate with various incident angles, spherical aberration is mainly generated in the luminous flux emanating therefrom.
Accordingly, it is difficult to perform accurate focusing in the optical pickup apparatus unless the refractive index or thickness of the optical transparent plate is set so that such spherical aberration becomes favorable.